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And the reason does because the diode


CHAPTER 4 The Real World


across it constant, an inductor wants to keep the current fl owing through it constant as well.


Let’s start with the load and work our way backward. To begin with, switch-ing supplies like to have a load. Without a load funny things can happen, but more on that later. What the load wants (in most cases) is a constant voltage. If I remember Ohm’s Law correctly, one can control the voltage across a resis-tor (i.e., load) by controlling the current through it, so let’s consider the fl ow of current in this circuit. We will begin with the switch closed. With the switch

Voilà, the process starts all over. There are two important things I noticed once the pieces fell into place in my head. The fi rst is that this control circuit I just described can be implemented with a simple comparator and a little hysteresis. Of course, that would lead to the frequency of the switcher being determined by the value of the inductor and the impedance of the load. That might or might not be a desirable trait. The other thing I realized was that when you fi rst turned it on, the circuit would want to slam the switch shut and keep it there for a long time while current builds up in the circuit. Are you beginning to see why switchers need a load?

Luckily, others much smarter than I have dealt with these problems already. That is why you hear terms like soft start and built-in PWM when you start studying switching supplies.

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and the reason does because the diode
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